The iPhone 4S will make its long-awaited debut this week. If you're planning to get one, make sure you check out Macworld's comprehensive buying guide first.

The iPhone 4S will launch on Friday 14 October and is available with three storage capacities - 16GB, 32GB and 64GB - and comes in two colours, black and white. When it comes to how much you'll have to pay and getting your hands on one, things start to get a bit more complicated.

Macworld has put together this comprehensive buying guide for the iPhone 4S, so if you're planning to take the plunge then read on to find out everything you'll need to know.

iPhone 4S specs and features

At a glance, the iPhone 4S looks similar to the iPhone 4, but on the inside is a very different beast. There's a dual-core A5 processor, the same seen in the iPad 2; dual-core graphics, offering up to seven times faster performance; 1GB of RAM, twice as much as in th iPhone 4; and the camera is 8MP (compared to 5MP in the iPhone 4) and shoots video at 1080p (compared to 720p).

In terms of software and services, it'll run on iOS 5, though only the iPhone 4S will be able to support the Siri personal assistant. It also supports iCloud, giving you online storage and backup for all of your important documents.

Pre-ordering an iPhone 4S

So now you've made up your mind that you want one, here's how to go about getting your hands on the iPhone 4S. Though launch day is a few days away you can pre-order the device through Apple and several (but not all) of the major UK mobile networks.

Vodafone, T-Mobile and Three will all let you pre-order an iPhone 4S through their respective websites (though Three will initially only sell 16GB models through its stores), whereas Orange requires you to phone its call centres. O2 has taken a slightly different tack, opting to make its stock available only to existing O2 customers who turn up at one of its stores on launch day.

SIM-free iPhone 4S

If you want to go SIM-free, you'll have to go through Apple. It'll cost you £499 for a 16GB handset, £599 for the 32GB version and £699 for a 64GB model. You can pre-order online, though Apple's site is currently quoting a wait of one to two weeks, meaning that you probably won't get one on launch day.

However, you can instead present yourself at an Apple Store on 14 October if you want to get your hands on one as soon as they are available. Be warned, though - 14 October is a Friday, so you might want to start begging and pleading with your boss right now, and doors will open at 8am. It's likely that people will queue overnight - one person has already started queueing at London's Covent Garden store - so even if you do go on the day, stocks might have dried up by the time you make it inside. Also - it's October. A wet, windy and cold month if ever there was one.

Contract iPhone 4S - picking the best deal

If you're after an iPhone 4S on contract - which, to be fair, is likely to be a more popular option given the price of the SIM-free versions - then you have a number of different options. All five major UK networks will stock the iPhone 4S, though getting your hands on one isn't guaranteed. As we've already mentioned, Vodafone, T-Mobile and Three are taking pre-orders through their sites but this is on a first-come, first-served basis.

There are dozens of different combinations on offer among the five networks so we've picked out a few of the most suitable for certain needs, but be sure to check out all of the plans on offer from each network before making your choice.

Best deal if you don't want to pay up front

If you want to get a free iPhone 4S you will have to make a few sacrifices - a 24-month contract and a high monthly cost. On Orange, you'll have to pay £46 a month for 24 months to get a fee 16GB handset, or £61 per month for the 32GB model. The 64GB model is not available for free on any Orange contract.

T-Mobile's best deal in this category is the £45.96 per month deal for the 16GB handset or £61.27 per month for the 32GB, again over 24 month periods.

Vodafone is offering fairly similar deals - on a 24-month contract you'll need to pay £46 per month to get a free 16GB handset, or £62 per month for a 32GB model. There is another option - an 18-month contract at £51 per month for a free 16GB model, or £67 a month over the same term for a free 32GB handset.

On Three, you can pay £43 per month for 24 months to get a 16GB handset for free (remember this is only available in Three stores) but the 32GB and 64GB models are not available for free on any contract.

O2 is offering a free 16GB iPhone 4S on its £41 per month 24-month contract, and a 32GB model for free on its £57 per month, 24-month contract. If you are an existing iPhone 4 customer with O2, though, you can get the 16GB iPhone 4S for no cost on a £36 per month, 24-month contract. If you only want an 18-month contract, you can get the 16GB model for free for £46 per month and the 32GB free for £62 per month. Again, existing iPhone 4 customers can get a slightly better deal, with the 16GB handset free on a £41 per month, 18-month contract.

However, O2 doesn't include data in these prices. So you'll need to factor in the cost of a data 'Bolt On' - 100MB for £3 per month, 500MB for £6 per month, and 1GB for £10 per month. The £3 per month deal doesn't cover Wi-Fi or MMS, though the £6 and £10 deals do, and all include tethering.

Best deal if you want a short-term contract

Only Vodafone and O2 are offering contract lengths shorter than 24 months. Both also have 12-month and 18-month deals available. A 12-month, £31 per month contract with O2 will get you 300 minutes of talktime and unlimited texts with O2, but you'll need to add at least £3 to the monthly cost for data. The upfront cost of this deal is £299.99.

Vodafone is offering a £36 per month, 12-month contract with 300 minutes, unlimited texts and 500MB data on the 16GB model with an upfront cost of £319.

Best deal if you want low monthly payments

O2's cheapest monthly deal is £15.50 per month over 24 months (remember - add at least £3 for data) and gets you 100 minutes of talktime and 500 texts. You'll need to pay £299.99 for the 16GB iPhone 4S model to get this contract, or £399.99 or £519.99 for the 32GB and 64GB models, respectively.

Vodafone's lowest monthly cost is £26 over 24 months , which includes 100 minutes, 500 texts and 250MB data and will require you to pay £359 up front.

On Three, £30 per month is the lowest you'll get and for the 16GB handset (in store only) it requires £129 up front, or £219/£309 for the 32GB/64GB models. It includes 300 minutes, 5000 texts and 500MB data and all deals are 24-months.

Orange has a £20 per month over 24 months deal for the iPhone 4S, giving you 50 minutes talktime, 50 texts and 100MB data. You'll have to pay £309.99 for the 16GB model, £449.99 for the 32GB model and £519.99 for the 64GB model up front.

Finally, T-Mobile has a £25.54 per month deal which will get you 100 minutes, 100 texts and 500MB over 24 months for an initial outlay of £239.99. For the 32GB handset, you'll pay £379.99 up front and £449.99 for the 64GB model.

Best deal if you want plenty of talktime

For unlimited minutes on T-Mobile, you'll need to sign up to the £61.27 per month package on any capacity iPhone 4S model - on the 16GB and 32GB models, this means the handset comes free. Orange's only deal with unlimited minutes is £61 per month. On O2, you'll need to sign up to at least a £57 per month contract for unlimited minutes, though remember that data costs extra.

Vodafone doesn't offer unlimited minutes, but you can get 3,000 minutes (50 hours) of talktime per month on its top deals. Three limits its contracts to a much more stingy 900 minutes (15 hours), though even the most garrulous of us are unlikely to spend that much time yapping on the phone in a month, unless you're a business user.

Best deal if you want unlimited texts

Just £21 per month over 24 months will get you unlimited texts on O2; with Vodafone it's a tenner more at £31 per month. With Orange, you'll also need to pay at least £31 a month to get unlimited texts. Three doesn't offer unlimited texts, but caps at 5,000. T-Mobile is even meaner, capping monthly texts at 500.

Best deal if you want unlimited data

With Three, £35 per month gets you unlimited data, and it is the only network that offers unlimited data packages for the iPhone 4S. O2's top data bolt on gives you 1GB, but you can buy more if you need them, Orange's £61 per month deal gives you a 2GB limit whereas you'll only get half that on Vodafone and T-Mobile's most costly deals.

What you'll need to change networks

If you're planning to switch networks and keep your old number you'll need to contact your current network to get hold of your PAC. This will need to be provided to your new network and be warned - it might take a couple of days to move your number over from one network to the other.

Buying an iPhone 4S on the day

It isn't just Apple Stores that will have stock in store for launch day - all the major network's stores will have a limited supply too, though in some cases (O2, for example), only existing customers will be able to buy a handset on launch day. And Apple is selling contract iPhones as well, not just SIM-free models.

Check the network provider's website to find out where your nearest store is and bear in mind it'll start at 8am on 14 October, so arrive in plenty of time. Best Buy, Phones4U and Carphone Warehouse stores are also an option if you can't make it to an Apple Store or one of your mobile network's outlets.

Other considerations

If you plan to buy your iPhone in person, be sure to take ID like a passport or driving licence with you, a proof of address such as a utility bill from the last three month, a recent bank statement as well as your PAC if changing network. You'll probably need to agree to direct debit payments, so take bank details - if you don't have a bank account it's unlikely you'll be accepted.